Trailing Headers

Originally opened by @tomchristie on 2020-08-08 10:49:55 in encode/httpx

One HTTP/1.1 feature that we don't currently support is trailing headers.

Hardly a high priority since it's seems so rarely used, but it might be worth our while someday.
It'd be interesting to hear from folks who do have use-cases here.

One option at the Transport API layer might be to update the AsyncByteStream/SyncByteStream APIs in order to handle this, without affecting the majority of code that doesn't expect or care about trailing headers, for example...

def get_trailing_headers() -> List[Tuple[byte, byte]]:
    # This method depends on the state of `__iter__`/`__aiter__`.
    # Once the byte stream has been fully consumed, any trailing headers may be accessed via this method.
    return []

At the httpx level we could expose this information by mutating the response.headers once the stream has been iterated over.

Start by reading the AsyncByteStream and SyncByteStream transport APIs, then trace how streamed responses expose headers. Explore the proposed trailing-header accessor and how response.headers could be updated after full stream consumption. Done means HTTP/1.1 trailing headers are supported without affecting streams that do not use them.